Growth Stimulants, Retardants,
and Rooting Hormones
Unit 5
Objectives
• List 1 example of a substance used to stimulate plant
growth
• Explain why chemical retardants are applied to floral crops
and name 2 commonly used retardants
• Explain the use of rooting hormones on cuttings and list
several rooting hormones
• Describe orally the source of 1 plant growth stimulant
• Demonstrate the proper application of a rooting hormone
to cuttings
• Explain how rootstock is used in dwarfing fruit trees
• Define biostimulants
Hormones
• Growth regulating substances in plants
• Hormones are organic chemicals that act and
interact to affect growth rate
• Auxins – (to increase) – accelerate growth by
stimulating cell enlargement
• Gibberellins – stimulate growth in stem and leaf
by cell elongation
– Also stimulate premature flowering, growth of young
fruits, and breaking of dormancy
Hormones (cont’)
• Cytokinins – stimulate cell division
– Work along w/ auxins
– Will not work w/o auxins present
• Inhibitors (abscisic acid) – inhibit seed
germination, stem elongation, and hasten
ripening of fruit
• Chemicals react w/ one another in complex
systems in the plant
Apical Dominance
• Terminal Bud secretes chemicals that inhibit
or prevent growth of auxiliary buds
– Causes the plant to grow tall rather than send
out side branches (p. 51 fig. 5-1)
– Helps in competition with other plants
• Methods other than pruning have been
researched to control plant growth rate, size
and shape
Stimulants
•
•
•
•
Enable plants to grow taller (p. 52 fig. 5-2)
Causes the stems to stretch out
Nodes are further apart
Example:
– Gibberellic Acid – may be natural or artificial
Chemical retardants
• Cause plants to be shorter and more
compact (p. 53 fig. 5-4)
• Used on lawn grasses (Limit)
• Interrupts cell division, stem elongation,
and seed head formation
• Roots continue to grow
• May reduce the natural Gibberellic acid
Rooting Hormones
• Used when propagating plants from cuttings
– Large percent of plants root and root quickly
• Usually mixed w/ talc and used as powders
or dissolved in liquid and used as a wet dip
• All rooting hormones should contain a
fungicide to prevent fungi from rotting of
the cutting
– (p. 54 fig. 5-7)
Dwarfing Rootstocks
• Developed to decrease labor costs in
orchards
• Rootstock – Root or piece of root used for
grafting
• Malling Rootstock – made it possible to
control the size and rate of growth of apple
trees by selecting the proper rooting stock
Plant Biostimulants
• Natural and Organic Product (from living
organisms)
• Work to stimulate soil microbial activity, stimulate
plant growth and promote disease resistance
• Humic acid (byproduct of Humus) is an example
• Methanol – blocks photorespiration so plants use
water for growth rather than transpiring it into the
air
Allelopathy
• The production of a chemical compound by
1 plant that slows down or stops the growth
of another plant
• Naturally used to stop competition from
other plants
• Some green manure or cover crops have this
effect on weeds
Objectives
• List 1 example of a substance used to stimulate plant
growth
• Explain why chemical retardants are applied to floral crops
and name 2 commonly used retardants
• Explain the use of rooting hormones on cuttings and list
several rooting hormones
• Describe orally the source of 1 plant growth stimulant
• Demonstrate the proper application of a rooting hormone
to cuttings
• Explain how rootstock is used in dwarfing fruit trees
• Define biostimulants